From the early beginnings of the automobile industry to the present, the most widely used engines use push rods which are actuated by a camshaft located in the engine block, adjacent and to the side of the crankshaft. Any scoring or damage to the camshaft bearings usually requires relacement of the bearings.
About 1968 a major change in engine design took place. The Triumph TR-7 and the Saab automobile introduced a cast iron block engine with an aluminum head and an overhead camshaft with non-replaceable bearings that are cast in as an integral part of the cylinder head. Scoring or damage to the cast-in camshaft bearings requires line boring of the bearing surface or replacement of the cylinder head. In 1975 VW, Audi and Porche adopted the overhead camshaft arrangement and later, several types of automobiles produced in Japan followed the overhead camshaft trend. Today, thousands of automobiles use the overhead cam engine design, and as the engines reach 80,000 to 90,000 miles in service, wear in the camshaft bearings reaches a point where reboring is required to give satisfactory engine reliability and performance. Present line boring tools are either too expensive, inadequate or take too much time to economically service these overhead cam engines.